afaik, these generators are unregulated. They are meant for filament bulbs, which are pretty tolerant of voltage variations. That said, if you ride fast, you will blow the bulbs. I think they pick a bulb/generator combo so that the bulb is normally run under-voltage at about 10mph. If you regularly ride faster than 15mph, you'll blow bulbs a lot.
One way round this problem is to increase the diameter of the generator wheel. Rubber bands wrapped round it can work. Of course you then have too dim a light a low speeds.
I'm not much of an electronics guy, but couldn't the unregulated output be fed to a capacitor? If it's actually an ac output, then a simple bridge rectifier should work, shouldn't it?
The capacitor output could go to a current source, such as a buckpuck, maybe with a voltage-limiting resistor.
How does that sound to you?